1. Field of the Invention
This invention relates to the field of digital audio and video compression, and, more particularly, to a system and method of compressing and decompressing digital audio and/or video information by subdivision into variable-sized packets followed by dynamic scaled quantization.
2. Description of the Related Art
Although the following discussion emphasizes digital audio storage and compression, the techniques described herein relate to digital video storage and compression as well.
Techniques of digital storage of audio and video information are well known in the art, in particular with regard to audio compact discs (CDs) and compact disc read-only-memory (CD-ROMs). Standards for storing audio information in digital form have been established (International ElectroTechnical Commission, International Standard--Compact Disc Digital Audio System, 1987, also known as "Red Book"). Digital compression reduces the data volume required to store digitally-encoded source material on a disc, thus providing many advantages, including facilitating storage of large amounts of such source material on limited data storage media. This is useful, for example, for storage of both video and audio information on common media.
Many digital compression techniques exist in the related art. The following examples of existing compression systems apply primarily to digital audio information, although they may also be used for video compression. Further examples and more detailed descriptions may be found in Parsons, Voice and Speech Processing, McGraw-Hill, Inc., 1987.
1) Reduced Sample Word Width: This technique involves discarding the less significant bits of each sample. Thus, for a sample of M bits, only the most significant N bits are stored on the disc, where N is less than M. Although this scheme is relatively simple, it results in an unacceptably low dynamic range (approximately 6*N decibels) and inferior signal-to-noise ratio.
2) Reduced Sample Rate: Here the signal bandwidth is reduced by lowering the number of samples per second. One major disadvantage of this scheme is the relatively expensive hardware required in re-sampling to a higher output rate.
3) Adaptive Differential Pulse Code Modulation (ADPCM): ADPCM generates a difference signal, which represents the difference between one sample and the next. Predictors are then derived and stored. ADPCM produces output of acceptable quality, but decompression is computationally complex. See Parsons, pp. 234-44.
4) Frequency Domain Compression: A frequency-domain analysis is performed on the signal to determine the frequencies at which the signal contains energy. Only the frequencies containing energy are stored. This technique is computationally complex, generally requiring random-access memory (RAM) for decompression. See, for example, Schwartz, U.S. Pat. No. 4,474,747 issued Sep. 18, 1984 for "Audio Digital Recording and Playback System ".
5) Adaptive Delta Modulation: A difference signal is developed as in ADPCM. Each difference quantity is scaled according to a step size that varies based on the nature of the signal. Decompression is relatively simple: the difference quantity is extracted by multiplying each compressed value by a multiplier. The multiplier may change from sample to sample; for example, the multiplier may increase after it has encountered two consecutive compressed values at maximum value. Each difference quantity is then added to the previous output value to provide a decompressed signal. Due to the lack of predictors, adaptive delta modulation facilitates simpler decompression hardware than ADPCM; however, this technique results in poor high-frequency response.
In general, most conventional compression techniques require relatively complex decompression schemes and expensive hardware.